Medium [CVE-2026-53385] fix null-ptr-deref in vcs_notifier during concurrent vcs_write

In the Linux kernel, the following vulnerability has been resolved: vc_screen: fix null-ptr-deref in vcs_notifier() during concurrent vcs_write A KASAN null-ptr-deref was observed in vcs_notifier(): BUG: KASAN: null-ptr-deref in vcs_notifier+0x98/0x130 Read of size 2 at addr qmp_cmd_name: qmp_capabilities, arguments: {} The issue is a race condition in vcs_write().

When the console_lock is temporarily dropped (to copy data from userspace), the vc_data pointer obtained from vcs_vc() may become stale. After re-acquiring the lock, vcs_vc() is called again to re-validate the pointer.

If the vc has been deallocated in the meantime, vcs_vc() returns NULL, and the while loop breaks (with written > 0). However, after the loop, vcs_scr_updated(vc) is still called with the now-NULL vc pointer, leading to a null pointer dereference in the notifier chain (vcs_notifier dereferences param->vc).

Fix this by adding a NULL check for vc before calling vcs_scr_updated(). A race condition in the `vcs_write()` function can lead to a null pointer dereference.

This occurs when the `vc_data` pointer becomes stale during concurrent write operations, and a subsequent call attempts to use the now-invalid pointer. A local attacker could exploit this vulnerability to cause a system crash, resulting in a Denial of Service (DoS).